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Preparation of Aldehydes and Ketones

Synthesis from Alcohols, Hydrocarbons, and Specific Named Reactions.

By chemca Team • Updated Jan 2026

Aldehydes and Ketones contain the carbonyl group ($>C=O$). While some methods prepare both, there are specific methods exclusive to aldehydes or ketones.

1. From Alcohols (General Methods)

Oxidation & Dehydrogenation

A. Oxidation: Use of mild oxidizing agents like PCC (Pyridinium Chlorochromate) converts primary alcohols to aldehydes and secondary alcohols to ketones.

$$ R-CH_2OH \xrightarrow{PCC} R-CHO $$ $$ R-CH(OH)-R' \xrightarrow{CrO_3} R-CO-R' $$

B. Catalytic Dehydrogenation: Passing alcohol vapors over heated Copper ($Cu$) at 573 K.

$$ R-CH_2OH \xrightarrow{Cu, 573K} R-CHO + H_2 $$ $$ R-CH(OH)-R' \xrightarrow{Cu, 573K} R-CO-R' + H_2 $$

2. From Hydrocarbons

Ozonolysis & Hydration

A. Ozonolysis of Alkenes: Reductive cleavage with $O_3$ followed by $Zn/H_2O$.

$$ R-CH=CH-R \xrightarrow{1.O_3, 2.Zn/H_2O} 2R-CHO $$

B. Hydration of Alkynes: Reaction with $H_2SO_4$ and $HgSO_4$. Ethyne gives Acetaldehyde; all other alkynes give Ketones.

$$ CH \equiv CH + H_2O \xrightarrow{Hg^{2+}/H^+} CH_3CHO $$ $$ CH_3-C \equiv CH + H_2O \xrightarrow{Hg^{2+}/H^+} CH_3-CO-CH_3 $$

3. Preparation of Aldehydes (Exclusive)

Named Reactions

A. Rosenmund Reduction: Reduction of Acyl chloride with $H_2$ over $Pd-BaSO_4$ (poisoned).

$$ R-COCl + H_2 \xrightarrow{Pd/BaSO_4} R-CHO + HCl $$

B. Stephen Reaction: Reduction of Nitriles with $SnCl_2/HCl$ followed by hydrolysis.

$$ R-CN + SnCl_2 + HCl \rightarrow R-CH=NH \xrightarrow{H_3O^+} R-CHO $$

C. Etard Reaction (For Benzaldehyde): Oxidation of Toluene with Chromyl Chloride ($CrO_2Cl_2$).

$$ C_6H_5CH_3 + CrO_2Cl_2 \xrightarrow{CS_2} \text{Chromium Complex} \xrightarrow{H_3O^+} C_6H_5CHO $$

D. Gattermann-Koch Reaction: Benzene with $CO$ and $HCl$ in presence of anhydrous $AlCl_3/CuCl$.

$$ C_6H_6 + CO + HCl \xrightarrow{Anhyd. AlCl_3} C_6H_5CHO + HCl $$

4. Preparation of Ketones (Exclusive)

Specific Methods

A. From Acyl Chlorides: Reaction with Dialkyl Cadmium ($R_2Cd$).

$$ 2R-COCl + R'_2Cd \rightarrow 2R-CO-R' + CdCl_2 $$

Note: Grignard reagents are not used directly as they further react to form alcohols.

B. From Nitriles: Reaction with Grignard Reagent followed by hydrolysis.

$$ CH_3-CN + C_6H_5MgBr \xrightarrow{Ether} \text{Adduct} \xrightarrow{H_3O^+} C_6H_5-CO-CH_3 $$

C. Friedel-Crafts Acylation:

$$ C_6H_6 + R-COCl \xrightarrow{Anhyd. AlCl_3} C_6H_5-CO-R + HCl $$

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